As technology advances, it becomes increasingly difficult and costly to test or verify the operation of electronic devices. One example of this is found in storage systems within the Hard Disk Drive (HDD) industry. There is pressure to reduce cost in the manufacturing of HDD subcomponents while not increasing the cost of scrap that occurs if failed subcomponents are not screened early enough and are not detected until later in the manufacturing process.
It is therefore desirable to test HDD subcomponents, such as the Flip Chip on Flex (FCOF) subassemblies used on the HDD Head Stack Assembly (HSA) early in the manufacturing process to increase yield and reduce cost. The FCOF is an electrical flex circuit assembly including an HDD preamplifier chip, such as products supplied by, e.g., Texas Instruments, a connector, and various other electronic components all electrically bonded to the flex circuit. This flex circuit will also have several bondpads available for electrical connection to one or more magnetic recording heads, referred to as Head Gimbal Assemblies (HGAs). For each HGA that will be electrically bonded to the flex circuit, the flex circuit will have a corresponding set of individual bondpads matching both the quantity and geometry of the bondpads of the HGA. Today's HGAs typically have 8 bondpads, for example, thus the flex circuits will contain matching sets of eight bondpads (i.e. bondpad sets) for each HGA to be bonded. As an example, considering an HSA designed for 8 HGAs, where each of these HGAs requires 8 bondpads, there will be 8 bondpads/HGA×8 bondpad sets, i.e., 64 individual bondpads per FCOF. For purposes herein, individual bondpads will be referred to as bondpads, and a bondpad set will be a set of these individual bondpads (for example, 8 bondpads) corresponding to the quantity and geometry used to match to the mating HGA.
During the HDD manufacturing process the FCOF will be installed onto an HSA Arm and this new assembly of FCOF plus HSA Arm is generally referred to as an Actuator Pivot Flex Assembly (APFA). Then, the one or more HGAs will be mounted to this APFA, through the combination of being mechanically swaged to the HSA arm and electrically bonded to the FCOF by bonding the HGA contact pads with the corresponding bondpad set on the FCOF to provide electrical connection. Then this HSA assembly will be installed into the hard disk drive for actual operation. Because the HGAs add significant cost to the HSA assembly, if the FCOF has any sort of failure, such as any opens or shorts in the traces on the flex circuit, any failures of the preamplifier chip, or any improper electrical bonding of any one of the bonded electrical components, it will be very costly to find the failure in the FCOF at later stages in the manufacturing process.
For illustration, reference is made to FIG. 5, which shows an example of an APFA 180, with an FCOF 182 and an HSA Arm 184, just prior to bonding 3 HGAs 186. In this example, each HGA 180 has 6 bondpads, so the illustrated FCOF 182 is shown with 18 bondpads 112 in total, in three sets of 6. Also illustrated on the FCOF 182 are a preamplifier chip 114 and a connector 111 bonded to a flex circuit 183.
The 6 or 8 HGA pad connections of each bondpad set provide unique functions to the magnetic recording head. One pair of connections will be the +/− connections for the magnetoresistive Reader element of the head. Another pair will be the +/− connections for the Write element of the head. Other individual or pairs of connections per each HGA will have other purposes, such as for flying height control, microactuation, or other features. As the hard disk drives become more complex the HGAs are designed with additional devices to increase performance, and so the quantity of individual bondpad connections increases because each of these new devices must be electrically coupled to and controlled by the preamplifier chip or primary circuit board on the hard disk drive. Thus, there is a tendency for the quantity of bondpads in each bondpad set to increase. Further, as the quantity of bondpads increase there is a tendency to make each individual bondpad smaller, for size reduction.
Current testers for FCOFs have test circuitry that can exercise the FCOF plus a means of electrically coupling this test circuitry to the FCOF, generally through the use of small probes. One set of larger probes, such as pogopins, will contact to the fairly large connector on the FCOF sample, and another typically smaller probe set will contact the individual bondpads sets on the flex circuit. Generally, due to the small size of the bondpads, these smaller probe sets use individual cantilever needle-probes, such as those provided by SV Probe, Inc., of Gilbert, Ariz., or other typical semiconductor probe card suppliers. After making electrical contact to the bondpads (input) and connector (output) of the FCOF, testers can measure the signals from the connector to determine whether the FCOF is operational by electrically simulating different head conditions on the bondpads. Testing is generally done by exercising the preamplifier chip through a series of test conditions. A typical test sequence is to simulate a Short condition across the +/− Reader bondpad pair, using the circuitry on the head simulation board, then measure if the preamplifier chip properly detects this Reader Shorted failure by looking at the resulting signals through the connector. Continuing the test sequence, the head simulation circuitry may then electrically float (open) these two Reader bondpad connections and confirm the preamplifier chip detects this Reader Open failure. This sequence may then be repeated for the Writer bondpad pair, and all other device bondpad connections. The head simulation circuit may further apply various typical loads to the bondpads, to simulate different possible Writer, Reader, and other device resistances, and verify that the preamplifier chip detects these normal conditions properly. This overall measurement practice is commonly referred to as Open Faults, Shorted Faults, or No Faults Detection. As an example, if the head simulation circuitry configured an Open condition on one pair of Reader bondpads, but the preamplifier chip responded as either Shorted or Normal condition, then this particular FCOF sample would be separated as having a failure. Various Fault and Resistance measurements are available through the preamplifier chip and are known in the art today.
Overall this system requires precision circuitry for simulating the different head conditions and also a precise means of probing each of the individual bondpads, generally in the form of needle-probes, which are quite small, delicate, and tend to dent, scratch, or otherwise damage the bondpads on the flex circuit. Therefore, it is desirable to make a simplified tester that can still detect these failures in the FCOF, but does not require precision probing or specialized head simulation test circuitry.
It is also desirable to test at the APFA level. Whether or not testing had been done at the FCOF level, the APFA testing will again purge failures that may have occurred before or after FCOF assembly, including any damage caused by mounting of the FCOF onto the HSA Arm. Per above the next step in the manufacturing process is to bond the HGAs to the APFA. The HGAs are a very expensive component of the HSA Assembly, so it is desirable to confirm the APFA and FCOF are functioning properly before the expensive HGAs are installed. Fundamentally, APFA testing is the same testing as FCOF testing, since they are both testing the same FCOF, except that the positioning requirements for probing the FCOF bondpad sets while the FCOF is mounted to the HSA Arm are very difficult. The difficulties are that the HSA Arm partially obstructs these bondpads, making access via cantilever needle-probes difficult, and the position of the FCOF is now more variable due to the mechanical tolerances of the HSA Arm that the FCOF has been mounted to, making tolerances higher and alignment much more difficult. No testers are known to exist today for this more complex application.